Parasitic mode filter



Nov. 12, 1968 P. BOUTELANT 3,411,116

PARAS ITIC MODE FILTER Filed July 30, 1965 2 Sheets-Sheet l In VEm-o Q ATTOQnEly Nov. 12, 1968 P. BOUTELANT 3,411,116

PARASITIC MODE FILTER Filed July 30, 1965 2 Sheets-Sheet 2 InvE I'TI'OQ. Pad BouRlant Amen Ey United States Patent 22 Claims. ((11. ass-es The invention relates to a mode filter associated with a multimode wave guide of a transmission line in which there is propagated a desired principal mode accompanied by a parasitic mode of higher order, the said mode filter having the effect of greatly attenuating the parasitic mode without substantially attenuating the principal mode.

Its principle consists in coupling a guide in which there are propagated a desired mode of order [m, n] and a parasitic mode of higher order [m, (n+p to an auxiliary guide length which is capable of transmitting a mode of order [m, q] having the same phase velocity as the parasitic mode of order [m,) 114+ p] in the principal guide, with the creation of a mode of order [m, (n.+p+q)] by superimposition, the energy contained in the mode [m, (n+p)] exciting the mode of order [m, q] in the auxiliary guide, in which the energy of the mode [m, q] is absorbed by matched absorbing terminations, while, since the phase velocity of the principal mode is ditferent from that of the auxiliary mode, the principal mode travels through the coupling length without appreciable attenuation.

The invention concerns more particularly a mode filter associated with a circular-wave guide of a transmission line in which there is propagated a desired cylindrical principal mode such as TE accompanied by a cylindrical parasitic mode of higher order, such as TE (Mp), of cylindrical form.

It is known that if waves of cylindrical TE n mode are passed through circular-wave guides, the mechanical imperfections of the guide, more especially minute differences in diameter at the junctions of the successive sections, have the eifect of creating modes of higher order such as TE (Mp). It is also known that the creation of parasitic modes constitutes an obstacle to a correct transmission of signals through a wave guide, more especially by the distortion which may result therefrom in the demodulation performed at the receiving end. It is therefore essential to eliminate such parasitic modes, while attenuating the principal mode as little as possible.

Mode filters for solving the above-mentioned problem have been proposed wherein the energy in a circular wave guide including both desired and spurious modes is filtered by coupling the selected desired mode to an outer coaxial waveguide section and terminating the main circular guide to absorb the remaining spurious modes. This is accomplished by interrupting the main guide and interposing a helix therein having a pitch and length necessary to couple the desired energy to the outer coaxial guide, which outer guide is dimensioned with respect to the inner circular guide such that only the desired mode will be coupled.

While these known mode filters are satisfactory in operation to a certain extent, they provide for an undesirable high attenuation of the desired energy through repeated couplings between guide sections which can never be carried on without loss of some energy. In addition, the provision of a helix which must be accurately designed to insure that the proper mode is coupled, along with other features of these devices, necessitates high manufacturing costs and limits the accuracy which may be achieved with such manufacture.

The filtering principle adopted by the invention consists in a guide transmitting a principal TE mode and at least one parasitic TE mode, in superimposing on 3,411,116 Patented Nov. 12, 1968 the cylindrical TE mode, a coaxial TE q mode by means of an external cylindrical conductor coaxial with the principal guide, forming an annular space called a coaxial guide around the principal guide, and coupled thereto by apertures in the common wall, in such manner that the energy contained in the parasitic mode passes substantially completely into the coaxial guide, in which it is absorbed by appropriate absorptive elements, the phase velocities of the waves of cylindrical TE (Mp) mode and of coaxial TE q mode being equal, while they are different between the principal cylindrical TE n mode and the coaxial TE q mode, so that, in the absence of interaction between these two latter modes, the wave of cylindrical principal TE mode passes through the mode filter without attenuation.

The condition of the above-described mode superimposition is that there should be, between the internal diameter D of the aforesaid coaxial external conductor and the internal diameter a of the principal guide (assumed to be very thin), the relation:

in which r (MPH) and r (Mp) are the zeroes of the Bessel function J (x), in which x is a normalized radial co-ordinate, of order (n +p+q) and of the order (n-i-p) respectively.

For this reason, in a guide for transmission in the cylindrical TE mode of internal diameter d according to the invention, there is provided a filter for a cylindrical parasitic TE mode which does not attenuate the TE mode, by disposing along a certain length a cylindrical conductor coaxial with the principal guide, of which the internal diameter bears, in relation to the internal diameter d of the guide of the transmission line, the above-defined ratio,

the coaxial line thus formed being coupled to the principal cylindrical guide by apertures in the common wall, which is assumed to be thin, and being closed at its two ends by matched loads.

Generally speaking, the diameter of the principal guide in the coupling zone may have a value d different from d the above condition becoming For obvious reasons of convenience in construction, it will be generally advantageous to give the mode filter of the invention an external diameter which does not exceed the dimensions of the principal cylindrical guide employed for the transmission.

Therefore, in accordance with a further feature of the invention, the central guide of the aforesaid mode filter has a diameter d smaller than the diameter d of the guide of the transmission line, the overall dimensions of the mode filter not exceeding the diameter of the guide of the transmission line, while the change from the diameter d to the diameter d takes place by way of conical transitions which are designed to ensure the purity of the principal mode as described, for example, in French Patent 1,142,543 applied for on Feb. 6, 1956, Device for Connecting Wave Guides of Circular Section Having Different Diameters.

In current applications, p will be equal to l and q will be equal to 1, and the above relation will be written:

2 11. (n+2) d '0, (n+1) Generally, n will also be equal to 1, and the above relation will be used in general in the form:

The coupling of the coaxial guide to the principal guide is effected, for example, by means of circular slots, so that all the energy of the cylindrical TE (n+1) mode present at the output of the mode filter is completely transmitted into the coaxial guide and absorbed in the matched terminations with which it is provided. At the same time, very little energy of the principal TE mode is transmitted into the coaxial guide, so that the insertion losses of the mode filter of the invention are very low.

The band widths of the mode filter of the invention is fixed by a low frequency F and a high frequency F These frequencies will be evaluated in the case where the principal mode is TE, 1 and the parasitic mode TE,

The low frequency has an upper limit which is the cutoff frequency of the mode TE 2 in the principal guide, that is to say, F equals c r /1rd, being equal to the velocity of light.

The frequency F is the cut-off frequency of the coaxial mode TE 2 which mode is likely to be reconverted into the cylindrical TE 1 mode in the principal guide and to give distortions, as will hereinafter be shown. F has the value c r /1rd. This relation will be established in the following.

The appearance of waves other than the coaxial TE 1 wave in the coaxial guide is harmful to the operation of the mode filter to the extent that it brings about the excitation of a parasitic TE, 1 mode in the principal guide.

Two cases are possible:

(1) The parasitic wave of the coaxial guide has the same phase velocity as the TE 1 wave of the main guide with a helical coaxial guide and a coupling of revolution. The power of the wave converted into TE 1 is extremely low.

(2) The wave of the coaxial guide TE q for example q l, is coupled to a TE wave of the principal guide which, in turn, may be reconverted into the principal TE wave in being propagated through the transmission. This coupling is impossible if the filter of circular electric TE (n+1) mode is at cut-off for the TE (n+2) wave. In the particular case where the principal mode is the TE 1 wave, and the wave to be eliminated is TE the guide of the diameter d will have to be at cut-off for the T13 wave. There is deduced from this the value of the frequency F which is the upper operating limit of the filter.

The above-defined relative band width is therefore, in the most unfavorable case, equal to:

and in the most favorable case, equal to:

If absolutely necessary, having regard to the mode discrimination between the TE 1 or TE 2 waves of the coaxial guide and the TE 3 wave of the principal guide, this upper limit may be extended to the cut-off frequency of the TE 4 wave of the principal guide which, in turn, is tightly coupled to the TE wave of the coaxial guide, because we have the relation The possible relative band width F /F then changes from 2.66 to 3.47.

These values amply cover the requirements of a telecommunications system operating with a circular-wave guide.

However, should the above-defined relative band width be insufficient, the invention provides, in accordance with further features, various means capable of widening it by varying the phase velocity of the coaxial TE mode along the coupling zone, for example by inserting into the coaxial guide a dielectric lining of variable thickness or dielectric strips of variable thickness.

Other means consist in providing around the aforesaid coaxial guide, along the entire coupling length, a second coaxial guide concentric with the first or an absorptive medium coupled to the first coaxial guide.

In accordance with a further feature of the invention, the coupling zone will be given a length which is so short that the energy of the cylindrical TE 2 mode cannot revert to the cylindrical TE 1 mode by way of the coaxial TE 1 mode, because such energy reconverted into the principal mode would be out of phase in relation to the principal mode and would therefore apply a distortion thereto.

In accordance with another feature of the invention, in a transmission line comprising a circular-wave guide, equipped with repeaters and mode filters, the mode filters are distributed in a variable manner between two repeaters, with a constant or nonconstant spacing, in order to benefit to a maximum by the compensation of the effects of these filters on the principal wave.

The invention will be described in detail with references to the figures, in which:

FIGURE 1 is a diagram illustrating the superimposition of the modes;

FIGURE 2 is an overall view of the mode filter av cording to the invention; and

FIGURES 3a and 3b illustrate an end section and a side section, respectively, of the construction of the mode filter in accordance with one embodiment of the invention.

FIGURE 1 is a transverse section through the mode filter of the invention, comprising two cylindrical conductors of diameters D and d curves indicating the relative amplitudes of the magnetic fields of the various modes in the particular important case in which the cylindrical principal mode is TE and the cylindrical parasitic mode is TE, The curve 1 represents the variation of the lfield as a function of the radius x normalized for the cylindrical TE 2 mode. This curve has the form of the Bessel function I with the root r 2 for x=d/2. The curve 2 is the variation of the field in the annular space between the cylinders of diameter d and D in ac cordance with the coaxial TE, 1 mode, the superimposition of these two modes giving rise to the cylindrical TE 3 mode, characterized by the root r 3 of the Bessel function 1 for x=D/2. Since the diameters are in the ratio D/d= o. 2

there is an interaction between the modes which have the same phase velocity. Consequently, the energy contained in the TE 2 mode enters the annular space in which it is absorbed by appropriate devices.

The construction of FIGURE 1 is justified by the fact that it is always permissible to make a conductive surface coincide with a surface of zero field; this is the case with the cylinder of diameter a of FIGURE 1. Of course, the thickness of the cylinder has a finite value; it must be relatively great as compared with the skin thickness, and small as compared with the wave-length.

Curve 4 represents the variation of the field in accordance with the cylindrical principal TE 1 mode. It will be seen that there is no interaction in the annular space with the principal cylindrical mode. The latter therefore passes through the mode filter without being attenuated.

In FIGURE 2, the mode filter of length L consists of the combination of a guide section 17 of diameter d, comprising circular coupling slots 5, of an external cylindrical conductor 18 of internal diameter D, preferably consisting of a helically wound insulated conductive wire, the annular space between the conductors 17 and 18 constituting a coaxial guide closed at both ends by two matched terminations 13 and 14 in the form of a conical tore, for example of graphite-loaded Araldite, and plugged by two conductive Walls 15, 16.

A principal T E 1 wave entering by way of a section 1 of the circular transmission guide having an internal diameter d passes without being affected through a first conical transition 2, a guide of diameter d forming part of a mode filter, and a second conical transition 6, and leaves by way of a section 7 of the circular transmission guide of diameter d A parasitic TE 2 wave entering by way of the section 1 of the transmission guide passes through the conical transition 2 and enters the mode filter in which it is trapped in the coaxial guide and absorbed by the termination 13. No energy can leave or enter the coaxial guide 20 because there is a conductive wall 15, 1'6 respectively behind each of the matched terminations 13 and 14.

Of course, in accordance with the principle of reciprocity, any parasitic TE 2 wave entering by way of the section 7 of the principal guide is absorbed by the termination 14, while a principal TE wave entering through the same section arrives at the section 1 undisturbed, since it has been assumed that the conical transitions have the desired purity of mode.

FIGURES 3a and 3b illustrate two sections of a preferred type of embodiment of the mode filter of the invention, a being a transverse section and b a longitudinal section. A dielectric tube 8 is internally coated with a conductive layer forming a series of rings 9 separated by slots 5 of width e.

The external conductor of the coaxial guide may be solid. It is preferably constructed in accordance with the helical guide techniques, since as is known, such a construction has a selective filtering effect on a circular electric wave. In FIGURE 4, it comprises a thin, helically wound insulated wire 10 secured to the interior of a steel tube 12 by way of a dielectric 11.

The dielectric 11 may optionally be separated into two parts by a fine-meshed metal gauze 13, forming a very regular cylinder in order to improve the elimination of certain parasitic waves, as is known :from French Patent 1,254,535 applied for on January 19, 1960, Improvements in Helical-Walled Wave Guides.

If the operating band width of the mode filter of the invention should prove insufficient, it could be increased, as has been stated in the foregoing, by employing a coaxial guide having a variable phase velocity in the coupling zone. This phase variation may be obtained by varying the thickness of a dielectric medium 19.

It may also be obtained by inserting conductive obstacles (for example radial rods) different from one another along the coupling zone.

Another possibility of widening the band width of the circuit is afforded by absorption of the coaxial TE 1 mode as it appears in the coaxial guide. Such a result may be obtained by one of the following means:

(a) Elements of albsorptive material are disposed in the coaxial guide along the coupling zone.

(b) A second coaxial guide concentric to the first is formed, in which the energy of the TE 1 mode is transferred as it appears. This second coaxial guide may be formed, for example, of the annular space situated between the metallic envelope 12 (FIGURES 3a and 3b) and the wound wire 10.

(c) There is disposed around the coaxial guide an absorptive medium coupled to the coaxial guide by coupling slots; the energy of the T-E 1 mode is thus absorbed as it is set up.

The examples given concern a circular guide and transmission systems employing circular-wave guides but it is to be understood that the coupling of an auxiliary guide to a principal guide, which effects by mode superimposition the transfer of energy of a parasitic mode from the principal guide into the auxiliary guide, in which it is absorbed by matched terminations, may be applied, without departing from the scope of the invention, to noncircular guides, for example sectorial or rectangular guides.

I claim:

1. A parasitic mode filter for a transmission line including a principal cylindrical wave guide for transmitting a principal mode of order [m, n] and within which at least one parasitic mode of order [m, (n+p)] is transmitted comprising guide filter means connected to said principal cylindrical wave guide for attenuating said parasitic mode including an inner guide section connected to said principal cylindrical wave guide for propagating said principal mode, said inner guide section having apertures in the outer wall thereof and an outer coaxial filter section coupled to said inner guide section by means of said apertures for absorbing said parasitic mode,

said outer coaxial filter section having dimensions capable of propagating a mode of order [mg q] higher than said principal mode having a phase velocity equal to that of said parasitic mode in the principal wave guide but difiFering from the phase velocity of the principal mode therein, and

matched absorptive terminations at either end of said coaxial filter section.

' 2. A parasitic mode filter as defined in claim 1 wherein said principal cylindrical wave guide has an internal diameter d transmitting a principal mode TE n and a parasitic mode TE said guide section and said filter section consisting of a thin lhollow internal cylindrical conductor of diameter d and an external hollow cylindrical conductor of diameter D, respectively, in coaxial relationship so as to form a cylindrical wave guide and a first outer coaxial wave guide,

the diameters of said external and internal cylindrical conductors having a ratio in which r t and ro,(n+p) represent zeros of the Bessel function J (x). r

3. Mode filter according to claim 2,}characterized in that the diameter d of the internal cylindrical conductor of the aforesaid mode'filter is equal to or greater than the diameter d of theprincipal wave guide.

4. Mode filter according to claim 2, characterized in that the external diameter of the aforesaid mode filter is at most equal to the diameter d of the principal wave guide and that the internal conductor of the anode filter, of diameter d less than d is connected to the transmission guide by two conical sections of a type which ensures purity of the principal mode.

5. Mode filter according to claim 2, characterized in that the internal diameter of the inner conductor of the mode filter is large in relation to the skin thickness thereof, and small in relation to the wave length of the energy transmitted therethrough.

6. Mode filter according to claim 2, characterized in that the aforesaid matched terminations are in the form of conical tores and constitute junction surfaces between the two aforesaid conductors.

7. Mode filter according to claim 2, characterized in that the aforesaid matched terminations consist of loaded plastic material. 7

8. Mode filter according to claim 2, characterized in that the annular space formed between the inner and outer conductors is plugged on either side externally of the aforesaid matched terminations, by conductive rings.

9. Mode filter according to claim 2, characterized in that the internal conductor is formed as a thin metallic deposit on a first dielectric tube.

10. Mode fi'lter according to claim 9, characterized in that the aforesaid metallic deposit forms conductive rings separated by circular slots.

11. Mode filter according to claim 10, characterized in that the aforesaid circular slots have uniform width over the entire coupling length.

12. Mode filter according to claim 10, characterized in that the width of the aforesaid slots varies along the coupling length for the purposes of ensuring optimum coupling effectiveness.

' 13. Mode filter according to claim 2, characterized in that the aforesaid internal conductor is a cylindrical metallic surface comprising preferably circular apertures.

14. Mode filter according to claim 2, characterized in that the aforesaid external conductor is formed by a helically wound insulated wire secured to the inside surface of a second dielectric tube, which is maintained by an external metallic sleeve.

15. Mode filter according to claim 14, characterized in that the thickness of the aforesaid second dielectric tube is subdivided by a fine-meshed conductive fabric wound in the form of a cylinder of very regular diameter.

16. Mode filter according to claim 2, characterized in that the annular space between the aforesaid two conductors contains a dielectric medium of variable thickness along the coupling length.

17. Mode filter according to claim 2, characterized in that the annular space between the aforesaid conductors includes metallic obstacles, whose shape, dimensions and distribution vary along the coupling length.

1 8. Mode filter according to claim 15, characterized in that means are provided to couple the aforesaid coaxial guide to the annular space situated between the aforesaid external conductor and the aforesaid metallic sleeve, the said space constituting a second coaxial wave guide in which the parasitic mode is propagated.

19. Mode filter according to claim 15, characterized in that the aforesaid first coaxial wave guide includes further absorptive elements disposed along the coupling length.

20. Mode filter according to claim 19, characterized in that the aforesaid second coaxial wave guide comprises absorptive elements disposed along the coupling length.

21. Mode filter according to claim 20 characterized in that the thickness of the aforesaid second dielectric tube is subdivided by a fine-meshed conductive fabric wound in the form of a cylinder of very regular diameter.

22. Mode filter according to claim 2, characterized in that the aforesaid guide filter means is of length L comprised between two limits, a lower limit such that the coupling therein is effective for the parasitic mode to be eliminated, and an upper limit such that the energy of the parasitic mode cannot re-enter the principal guide by way of a mode generated in said guide filter means.

References Cited UNITED STATES PATENTS 2,951,219 8/1960 Marcatili 333-21 3,184,695 5/1965 Unger 333-21 ELI LIEBERMAN, Primary Examiner. 

1. PARASITIC MODE FILTER FOR A TRANSMISSION LINE INCLUDING A PRINCIPAL CYLINDRICAL WAVE GUIDE FOR TRANSMITTING A PRINCIPAL MODE OF ORDER (M, N) AND WITHIN WHICH AT LEAST ONE PARASITIC MODE OF ORDER (M, (N+P)) IS TRANSMITTED COMPRISING GUIDE FILTER MEANS CONNECTED TO SAID PRINCIPAL CYLINDRICAL WAVE GUIDE FOR ATTENUATING SAID PARASITIC MODE INCLUDING AN INNER GUIDE SECTION CONNECTED TO SAID PRINCIPAL CYLINDRICAL WAVE GUIDE FOR PROPAGATING SID PRINCIPAL MODE, SAID INNER GUIDE SECTION HAVING APERTURES IN THE OUTER WALL THEREOF AND AN OUTER COAXIAL FILTER SECTION COUPLED TO SAID INNER GUIDE SECTION BY MEANS OF SAID APERTURES FOR ABSORBING SAID PARASITIC MODE, SAID OUTER COAXIAL FILTER SECTION HAVING DIMENSIONS CAPABLE OF PROPAGATING A MODE OF ORDER (M, Q ) HIGHER THAN SAID PRINCIPAL MODE HAVING A PHASE VELOCITY EQUAL TO THAT OF SAID PARASITIC MODE IN THE PRINCIPAL WAVE GUIDE BUT DIFFERING FROM THE PHASE VELOCITY OF THE PRINCIPAL MODE THEREIN, AND MATCHED ABSORPTIVE TERMINATIONS AT EITHER END OF SAID COAXIAL FILTER SECTION. 